OpenGL – Index buffers difficulties

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File types that use separate indices for vertices and normals do not match to the OpenGL vertex model very directly. As you noticed, OpenGL uses a single set of indices.

What you need to do is create an OpenGL vertex for each unique (vertex index, normal index) pair in your input. That takes a little work, but is not terribly difficult, particularly if you use available data structures. A STL map works well for this, with the (vertex index, normal index) pair as the key. I’m not going to provide full C++ code, but I can sketch it out.

Let’s say you have already read your vertices into some kind of array/vector data structure inVertices, where the coordinates for vertex with index vertexIdx are stored in inVertices[vertexIdx]. Same thing for the normals, where the normal vector with index normalIdx is stored in inNormals[normalIdx].

Now you get to read a list of triangles, with each corner of each triangle given by both a vertexIdx and a normalIdx. We’ll build a new combinedVertices array/vector that contains both vertex and normal coordinates, plus a new combinedIndices index list. Pseudo code:

nextCombinedIdx = 0
indexMap = empty
loop over triangles in input file
    loop over 3 corners of triangle
        read vertexIdx and normalIdx for the corner
        if indexMap.contains(key(vertexIdx, normalIdx)) then
            combinedIdx = indexMap.get(key(vertexIdx, normalIdx))
        else
            combinedIdx = nextCombinedIdx
            indexMap.add(key(vertexIdx, normalIdx), combinedIdx)
            nextCombinedIdx = nextCombinedIdx + 1
            combinedVertices.add(inVertices[vertexIdx], inNormals[normalIdx])
        end if
        combinedIndices.add(combinedIdx)
    end loop
end loop

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